Control/Dimming Multiple PWM Settings

Scott Lumley

Member
Joined
Sep 4, 2018
Location
Sault Sainte Marie, MI
I know pulse-width-modulation controls the on/off cycle of LED's, and lower Hz can be seen as flickering, but why not just use the highest setting? Not all lights let you change the PWM, but some do, like the Chauvet Professional Ovation B-2805FC. Is there a reason not to just use the highest setting? If not, why don't they just lock them into that setting? Is a setting in the middle better to use for some instances? Does a higher rate (or lower rate) effect how long the electrical components will last (or the LED itself)? Is there a magic number, that once passed is just overkill and unnecessary with modern cameras and the human eye?

I haven't worked with any lights that allow the changing of PWM yet, but hopefully soon, I will have some. I'm just looking for a better understanding of it. Thank you!
 

Harrison Hohnholt

Active Member
Joined
Dec 21, 2015
Location
Carlstadt, NJ
We work with PWM on our low voltage dimmers and LED tape.

One of our dimmers ranges from 500Hz to 30KHz. At the lower rate, you get much better low-end dimming. Especially, on slow fades to zero. And as we know theatrical designers love the slow fade to zero.

On the adjustable aspect, fastest might not always be best if it still flickers on camera. Being able to adjust fixes that problem. We have found being adjustable is better than just being able to be super fast.

I am not sure about how the electrical components are affected by the rate.

There is a magic number for the human eye, I am not sure what it is off the top of my head. For cameras, it depends since there are different max refresh rates and that setting can be adjustable.
 

Scott Lumley

Member
Joined
Sep 4, 2018
Location
Sault Sainte Marie, MI
Thank you both for your quick responses! I think I understand it a bit better now. Above a certain point (120Hz maybe) the human eye can't see the flickering. Depending on the camera though, some may need a higher rate, whereas others may need a lower rate (but still above the human eye).

So another question, sometimes when we do events that are recorded, the camera guy comes in last second (sometimes even when the event is suppose to start and we hold), and I might not have time to adjust the PWM or they might not even say anything to me about it, so is there a recommended setting that works for most cameras? If you had to pick just one PWM to go with, what would it be?
 

Harrison Hohnholt

Active Member
Joined
Dec 21, 2015
Location
Carlstadt, NJ
That one is tough for two reasons. Fixtures PWM varies in specificity and how many options there are available. Some have 4 some have 20.

The internet is saying 6KHz.
 
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Scott Lumley

Member
Joined
Sep 4, 2018
Location
Sault Sainte Marie, MI
I know one setting isn't always going to be the best choice, but I just want to know where I should start, and change if I notice or if someone else notices an issue. Thanks for helping me out with this, I really appreciate it!
 

Harrison Hohnholt

Active Member
Joined
Dec 21, 2015
Location
Carlstadt, NJ
I almost forgot. One thing I like to do to test is put my phone in slo-mo mode and see if it flickers. It won't catch everything but is a good starting point. It is always good to experiment too. My pixel gets up to 240 fps I haven't tested when I start seeing flicker on it though.
 

Scott Lumley

Member
Joined
Sep 4, 2018
Location
Sault Sainte Marie, MI
I didn't even think about using my phone in slo-mo mode, I will have to give it a try and play around with the settings to see when my phone can start seeing the flickering and when it can't. I also just noticed that I have a setting I can turn on after taking a slow-motion video that supposedly stops flickering. I'll have to play around a bit with both the lights and my phone.
 

Chris Pflieger

Well-Known Member
Joined
Aug 26, 2016
Location
Indiana
120 Hz seems really slow... I'm sure it varies from person to person, but for me, Cadillac tail lamps really drive me nuts because of their low rate.

At higher rates, the internal PWM resolution will be lower. Remember, the microprocessor is always running at a fixed rate, so if the PWM is 30 kHz, and the MCU clock is 72 MHz (mid-level ARM core), that's only 2400 steps available, or roughly 11 bit dimming. To get a full 16 bits, you would need to be ~1 kHz or lower.


I wonder if power consumption, radiated RFI, audible noise, are a factor when moving up to very high PWM rates.
 

Scott Lumley

Member
Joined
Sep 4, 2018
Location
Sault Sainte Marie, MI
In general, it sounds like the lower the PWM setting, the better the dimming because I will have more bits. The higher the PWM setting, the less likely it will show up on a camera as flickering. So I should be picking the lowest setting I can, but making sure it is just high enough to avoid flickering on a camera that may be recording. This would allow me to have more bits for dimming, making it smoother, but also keep the camera people happy. Is this all correct?

If so, I think I finally understand why there are selectable PWM's and why the highest setting isn't always the best, it depends on the scenario and equipment being used to record. Thank you everyone that has helped me with this!
 

Ford

Sr Product Manager, Chauvet Professional
Joined
Oct 19, 2007
Location
South FL
Harrison hit the nail on the head with his description of his LED tape. The slower the refresh rate, the nicer looking the slow fades...but the more likely that it will flicker on camera.

We have noticed (based on our limited sample of lighting designers and engineers) that the more sensitive among us can see the flicker at 300hz (especially out of the corner of your eye), and we have one guy who can see flicker up to 900hz... but he's not in the norm.
Most cameras used to work at frame rates that are a multiple of 50 and 60 hz. Back in the olden days, when dinosaurs were just starting to use professional LED pars, All of our COLORados used to be 600hz.
Then cameras got "higher tech." Now, most of our fixtures can go as low as 600hz, default at 1200hz, and have settings that hit various points (not always multiples of 50 and 60 anymore) all the way up to 25khz.
We have found that 1200hz works for almost all of our customers (even the television studios), but there is always that one guy who is filming a slo-mo Kung-Fu battle and needs something more specialized... so... options.

Also... The cameras (especially the ones on our cell phones) keep getting more and more advanced. Between Photo-Bursts, and the Slow Motion options that are becoming more and more prevalent, we want to include as many options as are practical.

Even the manufacturers who thought that their gear was future proof have found that technology waits for no-LED. I think that the Color Kinetics (or was it Color Force?) fixtures used to be set to 7200hz, which they claimed worked with all cameras... then it didn't. But that was back in like 2012... practically a lifetime ago.

I guess my point is that what used to be normal is now sub-standard. So, having options is the best way that we lighting manufacturers can keep ahead (or at least apace) with the dang cell phone manufacturers.
 

RickR

Well-Known Member
Joined
Sep 18, 2009
Location
Spokane, WA the great "Inland Northwest"
The architectural side has been getting serious about flicker for a few years. Google "flicker index" 120hz is very visible as much testing proves. https://www.lrc.rpi.edu/programs/solidstate/assist/pdf/LRC-Bierman-FlickerMetrics-2017-09-22.pdf and more at lrc.rpi.edu

The other point about having options is avoiding harmonics of the cameras shutter speed. 1200hz is great until the camera runs at 600 or 400! That guy doing super slo-mo can take a tip from 3 phase systems, set fixtures to a variety of speeds and they'll never harmonize into a problem. Just like pointing lights at a variety of angles.
 

John Palmer

Active Member
Joined
Apr 28, 2014
Location
Cerritos, CA
To kinda piggy back on what Ford mentioned, I just worked with some fixtures designed for camera that had a very high pulse rate and were "flicker free". The problem was that to get them to be "flicker free" the fixture had NO low end dimming. At DMX value 1, they snapped on at what visually seemed at least 20%. I programmed all intensity and color changes to happen in bumps because you could not smoothly crossfade from blue to magenta.
So just because a fixture is "flicker free" doesn't mean that it will work for a live event situation.
BTW, I didn't spec the fixtures. "They are really bright," they said. After working with a demo fixture, I said, "But they aren't really dim, which is what we need." IMG_0402.JPG
Take care,
John
 

Chris Pflieger

Well-Known Member
Joined
Aug 26, 2016
Location
Indiana
The architectural side has been getting serious about flicker for a few years. Google "flicker index" 120hz is very visible as much testing proves. https://www.lrc.rpi.edu/programs/solidstate/assist/pdf/LRC-Bierman-FlickerMetrics-2017-09-22.pdf and more at lrc.rpi.edu

The other point about having options is avoiding harmonics of the cameras shutter speed. 1200hz is great until the camera runs at 600 or 400! That guy doing super slo-mo can take a tip from 3 phase systems, set fixtures to a variety of speeds and they'll never harmonize into a problem. Just like pointing lights at a variety of angles.
Spread spectrum dimming - now there's something I haven't heard of yet.

Just thinking about writing the low level MCU driver code is giving me fantods.



FYI: I just dug into my current project, for a simple indicator LED, I used 4 kHz as the rate - a number completely taken from my backside.
 

sk8rsdad

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Ottawa
There are also several options for managing duty cycle. Most people talk about PWM and assume a repeating on-off cycle at a particular frequency. One technique called BAM chops up the cycle by slicing a cycle into a fixed number of time slices then going bit-at-a-time turning on or off for however many time slices the bit represents; that is, given the bit pattern 1011 the LED would be on for 1 slice + 2 slices then off for 4 slices, then on again for 8 slices. Other Simple variants include mirroring successive packets so they bookend each other. It's easier to explain with pictures but I don't have the time. Google PWM vs BAM if interested.
 

Chris Pflieger

Well-Known Member
Joined
Aug 26, 2016
Location
Indiana
There are also several options for managing duty cycle. Most people talk about PWM and assume a repeating on-off cycle at a particular frequency. One technique called BAM chops up the cycle by slicing a cycle into a fixed number of time slices then going bit-at-a-time turning on or off for however many time slices the bit represents; that is, given the bit pattern 1011 the LED would be on for 1 slice + 2 slices then off for 4 slices, then on again for 8 slices. Other Simple variants include mirroring successive packets so they bookend each other. It's easier to explain with pictures but I don't have the time. Google PWM vs BAM if interested.
Also called "Binary Code Modulation"...


interesting.
 

Pie4Weebl

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New York City
I was using some LEDs movers and there was high humm in the PA, I suspected it might be my lights causing it, so I changed the PWM to up above what a human can hear, low and behold the noise was gone!
 

Harrison Hohnholt

Active Member
Joined
Dec 21, 2015
Location
Carlstadt, NJ
I was using some LEDs movers and there was high humm in the PA, I suspected it might be my lights causing it, so I changed the PWM to up above what a human can hear, low and behold the noise was gone!
We get that with our low voltage dimmers. Almost like a singing. It is usually the Mosfets, I have found adjusting the load and/or PWM gets rid of the "singing". We do exactly what you did and just raise it above the human hearing range.